The present invention relates to a system for controlling an operation mode of a modulator/demodulator which is applicable to satellite communication. More particularly, the present invention is concerned with an operation mode control system for a modulator/demodulator useful for business communication and mobile communication of the kind using high frequency bands such as a Ku band, which is a promising high frequency band.
Satellite communication, compared to ground communication, requires a modulator/demodulator which is operable under lower signal-to-noise (SN) ratio conditions because it cannot avoid attenuation due to far longer transmission links. Especially, a demodulator for phase modulation adapted for synchronous detection has to positively tune a carrier recovery circuit at a sufficiently high SN ratio. One approach known in the art to meet this requirement is using a modulator switchover system which causes a modulator to perform two-phase PSK (phase shift keying) demodulation for a preamble signal and four-phase PSK demodulation for a transmit signal. Such a switchable demodulator scheme is disclosed in, for example, Japanese Patent Publication Nos. 53-23649 (U.S. Pat. No. 3,815,034) and 58-55709 which respectively are entitled "Phase-Modulated Wave Demodulator" and "Phase Demodulator".
The prior art systems will be described with reference to the accompanying drawings. In FIG. 1, a prior art system is shown and generally designated by the reference numeral 10. The system 10 comprises a demodulator 12, an envelope detector 14, a threshold detector 16, and a timer 18. A receive signal 20 which is applied to the system 10 has a format as shown in FIG. 2A. Output signals 22, 24 and 26 of the envelope detector 14, threshold detector 16 and timer 18, respectively, have waveforms as shown in FIGS. 2B, 2C and 2D. The demodulator 12 is selectively operable in different modes as shown in FIG. 2E. As shown in FIGS. 2A-2E, when arrival of a signal has been detected in a no-signal condition, the demodulator 12 is caused to operate in a two-phase PSK mode and then in a four-phase PSK mode. Another prior art system is shown in FIG. 3. In FIG. 3 the system, generally 30, comprises a demodulator 32 and a unique word detector 34. A receive signal 36 which enters the system 30 has a format as shown in FIG. 4A. The output signal 38 of the unique word detector 34 is shown in FIG. 4B, and the operation modes of the demodulator 32 are shown in FIG. 4C. It will be seen that in this particular prior art system 30 on detection of a unique word the operation mode of the demodulator 32 is switched from a two-phase to a four-phase PSK mode.
The problem with the prior art system 10 of FIGS. 1 and 2A-2E, which detects an envelope and then a threshold value, is that it is liable to perform faulty detection under low SN ratio conditions. Meanwhile, the prior art system 30 of FIGS. 3 and 4A-4E has the drawback that if it overlooks a unique word even once, the communication is practically disenabled from the start.
While any of the prior art systems discussed above allows the demodulator to operate in a two-phase PSK mode during an initial stage of operation and a four-phase mode during a steady communication stage, communication of the kind keenly demanded today for business transactions uses a modulator/demodulator which is operable in a far greater number of operation modes. For example, the following factors are used in various combinations:
______________________________________ Data rate: 4.8 Kb/sec 9.6 Kb/sec 32 Kb/sec Low 64 Kb/sec 1.544 Mb/sec Medium 10 Mb/sec 60 Mb/sec High 120 Mb/sec Modulation system: 2-phase PSK 4-phase PSK 8-phase PSK Error-correcting coding rate: 1/2 1/3 3/4 7/8 ______________________________________
It is remarkably effective to combine various modes as stated above so that one may hold a communication with an optimum modulation system selected on the basis of user's signal conditions (voice, data), SN ratio of the transmission path (attenuation due to rain, etc.), frequency band in use (Ku band, C band, etc.), and others. Although such may be implemented utilizing order wires of a TDMA (time division multiple access) communication system, installing order wires in business communication, mobile communication and others which involve numerous users is undesirable because the order wires would reduce the system capacity available for communication. Moreover, any of the prior art systems shown and described is incapable of controlling a modulator/demodulator having a great number of different operation modes as previously mentioned.